What's the best exercise to do to build up your butt? I've been doing lunges and squats. Is that the best? If so, how many times per week should I do it, and how many reps?

I would add dead lifts to it, and use as heavy weight as you can handle (I mean squats and dead lifts). generally , you shoudl work the same muslce group twice a week, to give it a complete time for recover, I would go for at least 48 hours. if you want to get gain muscles in this department, do 2 sets of 10 reps each with heavy weigths and a good form.
Good luck

Hmm... I don't belong to a gym, and I'm a pretty small girl, so I don't know how much I could dead lift anyway. When I'm doing squats, sometimes I do "body squats" (that is, if I'm prepared to have my entire lower body ache for the next week! :lol, or I hold one 3 pound weight in each hand. I also do a lot of light reps and stretching so that I don't get thick thighs. I heard doing that (many reps with low-weight) is how you build long, lean muscles, but in the same respect, I want to BULK-UP my butt, so maybe I do need fewer reps at higher resistancy.

Right now I do 100 walking lunges (ugh! Those KILL), and then 100 (body)squats. They're usually in 2 sets of 50, or 4 sets of 25 so that I really 'feel the burn'. But doing 2 sets of 10 sounds a heck of a lot nicer!

Thanks for the advice!

Squats, Deadlifts, and Leg Presses are good exercises that firm the butt and entire leg. Lunges are not so much a bulking exercise, but are a good shaping exercise from my understanding. Step-up really work the butt and are a great overall lower body exercise. Chose a height in which your knee is about 90 degrees when your other foot is on the ground. You can try these holding weights but be careful of your balance and with heavy weights make sure to keep you abs tight and do not round your back. I would be careful with step-ups if you have knee problems. A variation of step-up would be to climb stairs in a controlled fashion skipping every other step. A great benefit to this is it can be done with very limited equipment. Find a few flights and you got your exercise.

Another exercise is a glute kickback. You can do this if you have access to a machine that has a cable pulley with a foot harness. Basically put the harness over the middle of your foot, lean forward, and push your leg back. I have a Bowflex Powerpro, and this works great. This can also be done on a Universal Machine that has a leg press stations. The station where you sit in a seat and do a leg press pressing on the pedals. Rather than sitting in the seat, face the seat and brace yourself with your hands. One foot at a time push backward against the pedal. Make sure you start off light and get the hang of pushing backward through your heel. You need to be careful to keep your foot firmly on the pedal, or the whole weight stack could come crashing down. Some gyms have a fancy machine designed solely for glute kickbacks.

If you want to build up some bulk you are going to have to use heavier weights and lower repetitions. Don't worry about turning into a bubble-butt overnight. It takes a lot of time and effort to gain muscle mass. Unless you have the genetics and work out really hard you will probably tone up and fill out a little. Hopes this helps.

Brett